The esters of fatty substances are currently used in many applications as diesel fuels, household fuels, ecological solvents, basic compounds for the production of sulfonates of fatty alcohols, amides, ester dimers, etc.
In the case of diesel fuel, which today constitutes a large application for esters of fatty substances, a certain number of quite varied specifications have been established. The ester should not contain heavy compounds (triglycerides), few diglycerides and sterol esters, few monoglycerides, for example, less than 5%, few free fatty acids that can be corrosive, and little free glycerine, for example less than 0.2%, no strong acid or metal traces at all. This means that there is a specific protocol for obtaining the desired purity.
In the case of household fuel, it is clear that all these specifications are not always useful and are even sometimes counter-productive, but with the market for household fuel and that for gas oil often being merged, the specifications for household fuel resemble those of gas oil because in France, for example, household fuel can be used in farm tractors and construction-site equipment.
When an ester is produced from oil and monoalcohol, it automatically forms, depending on the nature of the oil that is employed at the beginning, 10 to 15% of a secondary product, which is glycerine. This glycerine is sold at a high price for varied purposes, but only when it is of high purity. The latter is obtained after purification steps that are carried out in specialized vacuum distillation units.
In short, most of the commercial processes for producing esters lead quite readily to raw products (esters and glycerine) which, however, have to be purified extensively by various treatments which ultimately raise the cost of the transformation.
Also, in the production of methyl esters of fatty substances from refined oils and dry alcohol, while simple alkaline derivatives, such as sodium, soda, or potassium alcoholates, are now used as catalysts, under rather mild conditions (temperature of 50 to 80.degree. C. and atmospheric pressure), as indicated in numerous patents or publications, for example, in JAOCS 61, 343-348 (1984), a pure product that can be used as fuel and a glycerine within specification can be obtained only after numerous stages.
If, for example, the most widely used alkaline catalysts are considered, traces of alkaline compounds are found both in the glycerine and in the ester, and it is necessary to remove them by washing the ester fraction and drying it. In the glycerine phase, it is necessary to neutralize the soaps and the alcoholates that are present, filter the salts that are formed, evaporate the glycerine after having eliminated the water, barring running the diluted glycerine over ion-exchange resins, before the salt-free glycerine is concentrated. Finally, it is always necessary to evaporate excess alcohol and often to distill it to keep this evaporation, mainly when it is carried out in the ester phase, from causing the ester that is present to react with the partially dissolved glycerine, which would lead to the formation of monoglycerides.
In short, to achieve the specifications that are desired for the glycerine and the ester, it is necessary to resort to so many stages that only large scale operation are economically viable.